Some invalid config value may cause BE work in an unexpected behavior,
this patch aim to support config validating when BE bootstrap and update BE's config by API
to reject invalid value.
This is a work to accomplish PR #4423
In the past, when we want to modify some BE configs, we have to modify be.conf and then restart BE.
This patch provides a way to modify configs in the type of 'threshold', 'interval', 'enable flag'
when BE is running without restarting it.
You can update a single config once by BE's http API: `be_host:be_http_port/api/update_config?config_name=new_value`
